Eastern Equine Encephalitis Virus in the United States: How Serious Is the Threat?

In the United States, health authorities have closed several public parks and issued warnings to the public following the death of a New Hampshire resident who had been infected with the Eastern equine encephalitis virus. The man, in his 80s, was hospitalized and died from this rare but potentially fatal mosquito-borne disease. What should you know about this condition, which has also been detected in other U.S. states?  

What is Eastern equine encephalitis?

Equine encephalitis (EEE) is caused by a virus generally found only in certain regions of the eastern part of the Americas, in an area stretching from Central America to Canada. It is an arbovirus (short for arthropod-borne virus), meaning a virus transmitted by arthropods (insects, ticks, spiders, etc.), in this case, mosquitoes. The EEE virus is transmitted to humans through the bite of an infected female mosquito. In some cases, the infection can cause serious neurological problems, including encephalitis (inflammation of the brain accompanied by edema—swelling). While most people bitten by a mosquito carrying the virus do not develop symptoms, some will develop the disease. These individuals then experience severe, potentially life-threatening symptoms: starting with headaches accompanied by neck stiffness and confusion, they can progress to seizures and coma. One-third of patients with severe symptoms will die, and many of those who survive will suffer from persistent neurological problems. Humans are not the only sentient beings susceptible to the EEE virus. As its name suggests, horses can also develop fatal encephalitis after being bitten by an infected mosquito. This arbovirus was first discovered in 1831, when a deadly epidemic spread among horses in New England, in the United States. The virus can also infect birds, with certain species being susceptible to the disease. Birds—whether wild, domesticated for farming, or living in captivity—are essential to the spread of the EEE virus, as they serve as its primary hosts.  

How does the Eastern equine encephalitis virus spread?

Birds, particularly passerines (a group that includes blackbirds, starlings, thrushes, and blue jays), play a key role in the virus’s transmission cycle. In fact, after infection, their bodies produce sufficient quantities of the virus that mosquitoes biting them become infected in turn. It is primarily female mosquitoes feeding on the blood of birds living in forested wetlands that enable the virus to circulate among bird populations. This is the case, for example, with mosquitoes of the species Culiseta melanura. It should be noted, however, that this mosquito rarely bites humans; its role in epidemics affecting human populations is therefore not significant. Transmission to horses and to humans is carried out by other genera of mosquitoes that feed on both birds and mammals. These include the genera Aedes, Coquillettidia, and Culex. Unlike birds, once infected, humans and horses do not spread the virus. This is because their bodies produce insufficient quantities of the virus for the mosquitoes that subsequently bite them to become infected.  

How can the spread be limited?

Currently, there is no treatment for Eastern equine encephalitis, and no vaccine is available for humans. However, a veterinary vaccine is available to prevent infection in horses. Limiting the population’s exposure to the virus therefore relies solely on personal protective measures and mosquito population control. In this regard, it is important to note that the impact of outbreaks can be significant, not only because of the disease itself but also due to the measures implemented to limit its spread and transmission. For example, recent outbreaks have led authorities in some cities to close parks in the evening and ask residents not to go out at night. Various events that were supposed to take place outdoors had to be canceled. Furthermore, the chemicals used to kill mosquito populations (via aerial spraying) are also a source of concern, as they are not specific to mosquitoes and can also be harmful to other insects.  

Why such an outbreak?

In tropical areas like Florida, the virus circulates year-round. In contrast, in the northeastern United States, its activity is more sporadic. Generally introduced to these regions by migratory birds, it is active there during the warmer months, when mosquito populations are expanding. The EEE virus has been known for many decades, and several outbreaks have been reported in North America. One of the most recent major outbreaks occurred in 2019 and affected several states in the northeastern United States. Thirty-eight people fell ill, and twelve died. While scientific studies indicate that the EEE virus is spreading to new regions of North America, future outbreaks are difficult to predict due to their sporadic nature. However, we know that a number of factors likely influence the virus’s spread into new geographic areas—and thus the risk of an outbreak. These include, for example, climate, various environmental factors, mosquito activity, and wildlife, among others. Not to mention human activities; this is an important factor, as public health risks increase with the growing frequency of contact between humans and infected mosquitoes, which can transmit the virus from birds that are also carriers. Urbanization, which sometimes encroaches on forested wetlands—habitats for mosquitoes and birds—could therefore increase the risk of an epidemic. In this context, health authorities must closely monitor mosquito activity and the viruses they transmit in order to be able to issue early warnings in the event of a potential human outbreak.

Is there a risk to other regions, such as Australia (or Europe)?

To date, the Eastern equine encephalitis virus has never been detected in Australia, and there is no risk of it being introduced there. However, it is important to note that mosquitoes of the genus Culiseta are indeed present in the country, and their role in outbreaks caused by local viruses (whether affecting humans or animals) is not yet fully understood. Furthermore, there are dozens of different mosquito species on this continent involved in the spread of alphaviruses (the group that includes the Eastern equine encephalitis virus, as well as other viruses circulating in Australia, namely the Ross River and Barmah Forest viruses). It is reasonable to assume that some of these mosquitoes might be capable of transmitting the EEE virus, but this hypothesis has not yet been tested in the laboratory. Finally, very little information is available on how local passerine species might play a role in establishing EEE virus transmission cycles in Australia. Nor are there any data on how vertebrates endemic to Australia (such as marsupials) would respond to infection. In any case, based on current knowledge, the risk that the EEE virus will reach Australia and spread there, causing outbreaks, is most likely very low. (According to Yannick Simonin, a professor of virology at the University of Montpellier and an arbovirus specialist, the risk of Eastern equine encephalitis emerging in Europe is currently considered low, but it is not zero: “Like other arboviruses, the virus responsible for Eastern equine encephalitis is monitored because there is a risk of its spread linked to global changes (climate change, international trade, population movements, etc.). In France, to my knowledge, there is currently no specific surveillance program for Eastern equine encephalitis,” (editor’s note ).  

Are travelers at risk?

The risk of being bitten by a mosquito carrying the Eastern equine encephalitis virus is extremely low. However, anyone visiting regions in eastern North America—including the United States and Canada—where encounters with mosquitoes carrying the virus may occur should take precautions to avoid bites. The most important thing is to follow the instructions of local health authorities and to make sure to avoid places (especially wooded wetlands) or times of day when mosquitoes are active (usually at dawn and dusk). Other recommendations include wearing loose-fitting, long-sleeved clothing and applying repellents (diethyltoluamide, more commonly known as DEET, picaridin, or lemon eucalyptus oil). These precautions will also protect you against infection by other mosquito-borne viruses you might encounter in these regions, such as West Nile virus. Cameron Webb , Clinical Associate Professor and Principal Hospital Scientist, University of Sydney , and Andrew van den Hurk , Medical Entomologist, The University of Queensland This article is republished from The Conversation under a Creative Commons license. Read the original article.